Generally, the main components of a postage meter comprise a top cover, a meter assembly and an electronic circuitry means usually in the form of a printed circuit board. Since these postage meters are subjected to transportation, use and storage in a multitude of environments, it is not uncommon for some condensation in the internal portion of the meter. For example, if the meter is carried from a cold environment to a warmer location, it is likely that water vapor will build up in the internal portions of the postage meter. This condensation or water vapor will in many instances come into contact with the power or circuit supply board and cause severe malfunctioning. In addition to this moisture problem, static electricity or charge is sometimes formed within the meter which also causes malfunctioning of the circuit board. Static electricity which builds up within the meter can result in a loss or distortion of data stored in the meter. This can result in improper operation of the meter including the possible improper accounting for postage which has been printed or which is available to be printed. A major problem in covers for electronic equipment relates to the fact that wiring must often be run to and from the electronic components through holes in protective coverings. These protective coverings are intended to protect the electronics where the device is subject to extremes in temperature and vibration.
There have been several attempts to eliminate or minimize this problem. One such procedure involves coating the circuit board with a conventional urethane spray. This however has not proven to be successful in totally eliminating the moisture problem. A more successful protective coating process involved Parylene vacuum coating the circuit board. This coating is able to penetrate beneath integrated circuits and other electronic elements to satisfactorily shield the sensitive board from moisture. These coatings are generally applied in about 1 mil thickness which seems to satisfy most moisture protection needs. This Parylene coating procedure for forming coating or protective shields for moisture sensitive electronics is described in an article in Electronics Packaging and Production, June 1984, pages 150-151. Unfortunately, this described procedure requires special preparation, specific application equipment and laboratory capabilities which take it out of the realm of typical manufacturing coatings and encapsulants. Items to be coated must first be cleaned and primed, followed by procedures which take into account a controlled thickness as well as conformal coverage. Its widespread use has been limited because of the complexity involved and the impractical nature of it. In addition, each time a different logic board is to be coated, new parameters must be determined, and the procedure must be radically adjusted.
Various protective shields have been used in the prior art especially those adapted for use with a circuit board. These protective devices are described in U.S. Pat. Nos. 3,845,253; 3,909,504; 4,308,953; 4,322,702; 4,386,388; 4,388,671; 4,394,707 and 4,399,487. In U.S. Pat. No. 3,845,253 an insulating block or shield is used but because of its non-conductive nature, it is of little use in dissipating static electricity. In U.S. Pat. No. 4,322,702 another protective shield or housing of insulating material is disclosed and as in U.S. Pat. No. 3,845,253 this type protector does not assist in disposing of unwanted accumulated static electricity. In U.S. Pat. No. 4,386,388 an aluminum sheet protector is disclosed but there is no provision therein for the venting of internally generated gases. In U.S. Pat. No. 4,394,707 the protection for the circuit package also involves the use of an electrically insulating cover for the electrical components. There is no provision for the venting of generated gases out of the package. U.S. Pat. No. 4,388,671 discloses a means to protect against electrostatic charges, however, no overall protection is provided for the electrical components. The shield described in U.S. Pat. No. 4,399,487 is an aluminum foil without provision for gas venting or moisture protection.
In U.S. Pat. No. 4,308,953 a protective device for electronics is disclosed which involves the use of a cardboard container which is conductive. The main purpose of this prior art container is to provide an electrically conductive means for grounding circuit boards and the like having static sensitivity planar objects. Devices such as postage meters easily pick up static charges in storage or transportation and unless dissipated, these charges will adversely affect the functioning of the meter. While the device disclosed in U.S. Pat. No. 4,308,953 effectively prevents the build up of static charges in a device containing electronic components, it is constructed of cardboard which provides little or no protection for the water vapor problem alluded to above. In U.S. Pat. No. 3,909,504 a moisture protective means is disclosed for electronic components. This prior art means involves the use of a polyethylene bag to surround the components. While appropriate moisture and temperature protection is afforded by these bags, heat is easily retained within the bag which could eventually degrade the circuitry and cause problems.
It is therefore clear that a protective cover for electronic circuit boards or the like should include moisture protection in addition to providing means to minimize the presence of unwanted electrical charges such as static electricity. The cover should be easily adapted to modern manufacturing methods, be versatile in use and provide for protection of the circuit board and, most important, venting of damaging heat generated during use. These needs are not totally provided for in known prior art devices discussed above.